Kevin Drum thinks the defection of 34 Republican Senators from the Norquist line doesn't matter much:

[M]aybe this is more of a good old sectional fight than a real schism on the proper interpretation of Norquist's anti-tax pledge.
We'll see. The theory here is that having once voted to end a tax expenditure (the ethanol subsidy), Republicans will now be more willing to defy Norquist and vote to end other, bigger tax expenditures (mortgage interest, employer healthcare contributions). I have my doubts about that. Sen. Jon Kyl of Arizona may have voted for the Coburn amendment, but he's also adamant that revenue increases remain off-limits in the debt ceiling talks. This vote has produced a lot of over-the-top rhetoric and frayed tempers, but in the end I suspect it doesn't mean much. Republicans remain just as firmly anti-tax in all its incarnations as they've ever been. Especially with an election coming up.

I think Drum's making a common mistake here. The mistake is his assumption that Senate Republicans just happened to abandon Norquist because they oppose ethanol, and so on this particular issue, they voted their opposition to ethanol over their opposition to taxes.

That isn't what's going on here. Virtually all the media coverage has gotten this vote wrong. Tom Coburn was not going about this in order to eliminate the ethanol subsidy. He made no attempt to work with the House, line up a majority, woo Senate Democrats, arrange for a vote on favorable terms, or do any of the things that Senators do when they're trying to pass a law. His goal was to do one thing: set a trap for Grover Norquist. He's been laying the trap since March.

The point of it is to establish a principle. Republicans working on a bipartisan deficit deal want to define the closing of tax expenditures as not constituting a tax increase. Their problem is that the Taxpayer Protection Pledge, which virtually all Republicans in Congress have signed, specifically defines closing tax expenditures as a tax hike. Coburn's ploy was a way of getting a foot in the door. That's exactly why Norquist is so enraged at Coburn.

Coburn's bill is not going to succeed and never had any chance of succeeding. So the question is, why did 34 Senate Republicans alienate Norquist by voting for it? If they wanted to take a public stand against the ethanol subsidy, they had a Norquist-approved amendment that they could have supported instead. The only reason to support Coburn was to help him undermine the Pledge.

Now, this hardly means the reign of anti-tax absolutism in the GOP is over. It remains the dominant party stance. But for the first time in two decades, there is a crack in the foundation.